The prior art discloses systems which measure differences in concentration levels of additives in a liquid or differences between two different liquid mixtures by measuring the heat capacity or the thermal conductivity of the liquids. Prior art systems measure the heat capacity of a liquid by measuring the temperature rise of the liquid as it flows past a heating element at a constant flow rate. Thermisters have been used in the prior art to provide an output signal which is a function of the specific heat, thermal conductivity and flow rate of the liquid flowing past them. Electrical current flowing through the thermisters heat them above the ambient temperature of the liquid flowing past. Since the liquid flow past the thermisters tends to cool them down, the temperatures of the thermisters reach equilibrium values which are proportional to the rate at which the heat is being transferred away from them by the flowing liquid. The output current of the thermisters are proportional to their internal resistance which in turn is proportional to this equilibrium temperature. Hot wire and hot film anemometers operate on the same principles as do thermisters and are frequently used to measure flow rates of fluids. In anemometers, a hot wire or hot film on the probe of the anemometer is heated by electrical current to a temperature above the ambient temperature of the fluid flowing past it, the internal resistance of this hot wire or film being proportional to its temperature.
The prior art teaches the use of thermisters and anemometers to detect changes in the thermal properties of liquids caused by additives in the liquids. However, these prior art systems have several limitations. Several of the prior art measurement systems do not maintain a constant flow rate past the thermister or anemometer and therefore are unable to distinguish between changes in flow rate and changes in the concentration level of additives in the liquid. None of the prior art measurement systems includes a means for sampling liquid from several different sources and sequentially measuring the additive concentration level in these different samples using a single sensor. The output signals of hot film anemometers or thermisters will also vary as a function of the ambient temperature of the liquid flowing past them, but none of the prior art measurement systems include means to compensate the output signals to remove the effects of changing liquid ambient temperature. The prior art measurement systems could not be used to measure the concentration level of additives in a liquid which change the viscosity, thickness of the laminar boundary layer or other turbulent flow characteristics of the liquid without changing the specific heat and heat conductivity of the liquid. The effects of such additives on the output signal of prior art system are very indirect and unpredictable.